Have you ever cooked a roast dinner? That's all it is. If you haven't ever made roast potatoes or gravy then you've got 12 weeks or so to have a bit of a practice.
You can buy everything in trays from M&S and just bing it in the oven but I'm not sure how many their smallest sizes feed so you could end up with lots of leftovers.
My personal preference for a couple of guests would be to buy either a small turkey crown or a breast roast that comes in an oven ready foil tray. They will both have timings on the label. The breast will (probably) say a total cooking time like 'Cook for x minutes' and the crown will say 'Cook for x minutes per 100g' and you will need to work out a cooking time.
You might want different meat - a better cut of beef or a nice leg of lamb.
Decide what time you want to eat and work backwards.
Say eat at 2. Meat is best with a bit of resting time before you carve it so aim to take it out of the oven 30 minutes before (so 1:30 in this example) and wrap in foil and cover with a tea towel to keep it warm.
Roasties take about an hour. Put oil/goose fat in your roasting pan and put in the oven to heat up while you parboil your spuds. Parboil the potatoes for 5-10 minutes, drain and give them a bit of a shake. Tip them into hot oil. I turn mine halfway through.
I do roasted carrots and parsnips in the same way as the spuds except they go in the oil without parboiling. Finish with a squeeze of honey.
I also do sprouts (microwaveable bag) but I finish them off in a Sauté pan with a bit of butter and some pancetta.
DH likes peas too but he's a heathen
Pigs in blankets are bought ready made and cooked according to the packet.
I make my own stuffing but Paxo is great or the supermarket will have some ready made.
Jamie Oliver's make ahead gravy is supposed to be brilliant but I don't like it and make my own. If you don't want to or don't feel confident then granules are fine and easy to use.
Christmas pudding goes in the microwave as per the instructions on the packet.
Don't stress. Cook the bits you like and are happy making and ditch everything else. I've made Christmas dinner for the last 30 years. It doesn't scare me now but I don't do elaborate side dishes as my family don't like bread sauce and red cabbage etc.